Our long-term objective is to understand mechanisms of directional hearing that involve the auditory processing of complex sounds. The spectra of complex acoustic stimuli are filtered by the external ear as part of the process of transduction in the auditory system. These filtering effects are manifested as peaks and notches in the amplitude spectrum of a broadband stimulus. Because the frequency regions that exhibit spectral features are influenced by stimulus location, this acoustic information is presumed to aid in sound localization. We prepose to describe the directionally-dependent acoustic properties of the mammalian pinna that shape the spectra of complex sounds. We also will isolate spectral features that arise from pinna-filtering properties to examine their functional significance in directional hearing. Finally, we will evaluate the role of neural networks within the dorsal cochlear nucleus for processing spectral features that convey meaningful information about sound location. Cats will be used as experimental animals in these studies because the acoustic properties of a cat's pinna have been investigated extensively, and because a wealth of physiological data exist on the neural basis of directional hearing in the species. Our behavioral experiments will assess the funcUonal significance of that acoustic and physiological evidence in animals that have been deafened in one ear. Unilaterally deafened animals must depend on monaural spectral cues to locate sounds in space. Once these experiments establish how spectral features contribute to monaural sound localization processes, cats with normal binaural hearing will be evaluated in future studies. Behavioral experiments using animal subjects will provide new insight into the neural processing of complex sounds in the auditory nerve and cochlear nucleus. This information could contribute toward the design of prosthetic devices for hearing unpaired individuals.